How Many Americans can the Earth Support?

Dr. David Pimentel, Cornell University

Based on the current growth rate, the present U.S. population of more than 270 million is projected to double to 540 million within the next 70 years. In addition, the world population -- about 6 billion -- is projected to double within just 50 years (again, based on current rates of growth). The growing imbalance between the increasing world population and the finite amount of Earth's resources that support human life is reason for grave concern.

Consider that according the World Health Organization, more than 3 billion people are currently considered malnourished. This represents the largest number and proportion of malnourished humans ever in history! Deaths from malnutrition and other diseases have significantly increased, especially during the past decade, and there is no indication that this trend will cease or reverse. What can we expect as population numbers continue to climb?

In order to support increasing numbers of people, we will need to be able to feed them. The production of adequate food depends on ample supplies of fertile cropland, pure water, energy, and other biological resources, like plants and pollinators. Growing numbers of humans, though, force us to stretch these limited resources further and further. The fact that grain production -- which supplies 80% to 90% of the world food -- has been declining since 1983 should alert us to the potential for future food supply problems and increasing malnutrition.

About 0.5 hectares (1.2 acres) of cropland per person is required to provide a diverse diet similar to that desired by the average American and European. At present, this amount of land is still available in the United States for its present population. In contrast, worldwide, only 0.27 ha of cropland per person remains for food production. Since land is a finite resource, available cropland per person will continue to decline, both worldwide and in the United States, as the human population increases. Urban sprawl, highways, and industries also spread and cover more land. Finally, a substantial amount of fertile cropland is lost to erosion by wind and water every year. Some 10 million hectares of cropland is being eroded and abandoned each year throughout the world.

Rainfall, as well as water captured in rivers and lakes, is essential for all plants, including crops. As agricultural production increases to feed more humans, pressure on water supplies also increases. Because communities, states, and countries must share water, competition for water resources increases. In arid regions of the world, which supply 30% of the world's food, irrigation has declined during the past decade. This has already had a negative impact on food production in these regions. Even in some areas of the United States, sufficient water for crops and people is becoming a serious problem. The fact that the great Agualla aquifer of central United States is being depleted about 140% faster than rainfall recharges it, suggests an impending serious water scarcity for a large area of U.S. land.

In addition to land and water resources, energy is also vital to crop production. Solar energy and human power, augmented with fossil energy, make the cultivation of crops possible. Fossil energy is used to power farm machinery and irrigation pumps as well as to produce fertilizers and pesticides. Unfortunately, fossil energy is a finite and non-renewable resource that is being rapidly depleted throughout the world.

Lastly, humans and their assorted activities are reducing biodiversity throughout the world. Pollination, essential for one-third of the world's food supply and dependent on diverse species of pollinators, has been declining; some U.S. crops already face serious problems due to lack of sufficient pollination. The stability of other essential biological resources for agriculture and forestry, such as microbes and invertebrates, are also declining and being threatened due to human activities. Finally, the use of more than 100,000 different chemicals -- including pesticides -- worldwide reduces vital biodiversity even further.

As increasing numbers of humans travel and trade more, more exotic species of plants and animals invade the U.S. and other ecosystems worldwide. Some of these exotic species become pests, which can increase food losses and frequently alter natural habitats. From 40% to 80% of agricultural pests are biological invaders, and -- despite the 5 billion pounds of pesticide applied worldwide -- more than 40% of potential food is destroyed by pests each year.

At present, humans face serious malnutrition, land degradation, water pollution and shortages, and declining fossil energy resources. In addition, with related changes in the natural environment, many thousands of species are being lost forever. If the human population increases dramatically over the next several decades, as it is projected to do, the strains on these limited resources will grow as well.

Some people are starting to ask just how many people the Earth can support if we want to cease degrading the environment and move to a sustainable solar energy system? There is no solid answer yet, but the best estimate is that Earth can support about 1 to 2 billion people with an American Standard of living, good health, nutrition, prosperity, personal dignity and freedom. This estimate suggests an optimal U.S. population of 100 to 200 million. To achieve this goal, humans must first stabilize their population and then gradually reduce their numbers to achieve a sustainable society in terms of both economics and environmental resources. With fair policies and realistic incentives, such a reduction in the human population can be achieved over the next century.

David Pimentel is a professor of ecology and agricultural science at the College of Agriculture and Life Sciences, Cornell University, Ithaca, NY 14853-0901. His Ph.D. is from Cornell University. His research spans the fields of basic population ecology, ecological and economic aspects of pest control, biological control, biotechnology, sustainable agriculture, land and water conservation, natural resource management, and environmental policy. Pimentel has published more than 490 scientific papers and 20 books and has served on many national and government committees including the National Academy of Sciences; President's Science Advisory Council; U.S. Department of Agriculture; U.S. Department of Energy; U.S. Department of Health, Education and Welfare; Office of Technology Assessment of the U.S. Congress; and the U.S. State Department.